Mutations in voltage-gated sodium channels have been implicated in some forms of human epilepsy, including the heterogeneous syndrome Generalized Epilepsy with Febrile Seizures Plus (GEFS+). Within GEFS+ families there is variable expressivity of the phenotype among family members with the same primary sodium channel mutations. The epilepsies seen in GEFS+ families with sodium channel mutations include almost all seizure types, ranging from only febrile seizures to temporal lobe epilepsy. This suggests that there are other factors that influence the clinical phenotype which may include genetic modifiers. Scn2a-Q54 transgenic mice have a mutation in the voltage-gated sodium channel Scn2a and are a model of inherited epilepsy. Consistent with the human patients, the clinical severity of the epilepsy in Q54 mice is influenced by the genetic background. Q54 mice on the resistant C57BL/6J background have delayed seizure onset, decreased severity, and increased survival compared to susceptible (SJL/J x C57BL/6J)F1 mice. This suggests that genetic modifiers underlie the susceptibility and severity of the phenotype. We have mapped two modifier loci that influence the seizure phenotype in Scn2a-Q54 mice. We will perform high resolution mapping of these loci and test positional candidate genes by BAC transgenesis. We will use genome-wide ENU mutagenesis to identify additional epilepsy modifier genes in a sensitized screen with severely affected Scn2a-Q54, Kcnq2 double mutant mice. Mutagenesis increases the pool of potential modifier genes and provides a complementary approach to QTL analysis which relies on existing variants between mouse strains. Finally, we are developing Scn1a knock-in mice carrying human epilepsy mutations and will test the effect of modifiers on these mice as they become available. The goal of this proposal is to identify modifier genes that influence the clinical course of epilepsy. Identification of epilepsy modifier genes will provide insight into the complex inheritance and pathogenesis of epilepsy, and suggest novel therapeutic targets for the treatment of human epilepsy.